The invention relates to a process for the removal of acid gases from a gas mixture containing methane. Mixtures of this type occur mainly as natural gases; in addition to other hydrocarbons they generally contain acid gases such as hydrogen sulphide (H.sub.2 S) and carbon dioxide (CO.sub.2). Natural gases usually also contain (in very small quantities) carbonyl sulphide (COS) and mercaptans.
Before natural gases can be condensed (for example for transport in the liquid state) or can be supplied as a gas to consumers via a pipe system, the acid gases, in particular H.sub.2 S, must be removed. Although some amount of CO.sub.2 is acceptable if natural gases are to be transported in the gaseous state, the removal of the CO.sub.2 down to a very low content is necessary if the gas is to be liquefied. Otherwise, deposition of solid CO.sub.2 during the cooling of the gas can give rise to clogging. In general, substantial removal of COS and mercaptans is required in view of the requirements imposed in respect of the sulphur content of the final gas. The generally light hydrocarbons (such as propane, butanes and pentanes) which are present in the gas are also preferably separated from the gas because they represent a high value as liquid or compressed combustibles, and because if they form liquid deposits in pipe systems, they impede the transport of the gaseous methane.
In order to achieve the removal of these components from the natural gas as economically as possible, it is desirable to obtain the H.sub.2 S in such a concentration that it can be processed to sulphur in a simple manner. This processing is generally carried out with advantage in a Claus process, in which H.sub.2 S is converted into sulphur and water by reaction with SO.sub.2. The most attractive manner of carrying out the Claus process is by supplying such a quantity of oxygen (for example, as air) to an H.sub.2 S-containing stream so that one third of the H.sub.2 S is converted into SO.sub.2 and the resultant quantity of SO.sub.2 is consequently just sufficient to be converted with the remainder of the H.sub.2 S into sulphur and water. This conversion can be carried out thermally and/or by means of a catalyst (for example, alumina). However, to carry out this process in the manner described, the quantity of H.sub.2 S in the gas to be processed must be at least about 40% by volume. For economic reasons, of course, it is desirable that the H.sub.2 S content of the gas to be processed is as high as possible. In the case of concentrations between 20 and 40% by volume, a Claus process is possible but special measures are necessary. In that case, one third of the H.sub.2 S must be separated from the gas stream, this portion must be separately converted with air into SO.sub.2, and this SO.sub.2, together with the remaining H.sub.2 S, must be passed over a catalyst, thus forming sulphur and water. In the case of H.sub.2 S concentrations below 20% by volume, even more complicated measures are necessary.